Abstract
Object agnosia is defined as an object recognition deficit and typically results from lesions of occipito-temporal cortex. However, little is known about the cortical (re-)organization of visual representations and, specifically, object representations in agnosia. We used fMRI to examine the cortical organization with respect to retinotopy and object-related activations in an agnostic patient and control subjects. Patient SM has a severe deficit in object and face recognition following damage of the right hemisphere sustained in a motor vehicle accident. Standard retinotopic mapping was performed to probe the organization of visual cortex in the lesioned and the non-lesioned hemisphere and to determine the lesion site relative to retinotopic cortex. Furthermore, we investigated object-selectivity in ventral visual cortex using fMRI-adaptation paradigms. Retinotopic mapping showed regular patterns of phase reversals in both hemispheres. Surface analysis revealed that the lesion is located in the posterior part of the medial fusiform gyrus anterior to V4 and dorsolateral to VO1/VO2. The contrast between object and blank presentations showed no significant difference in activated volume in SM, compared to healthy subjects. FMRI-adaptation induced by different types of objects, however, revealed differences in activation patterns. In healthy subjects, object-selective responses were found bilaterally in the anatomical location of the lesion site as well as posterior, dorsal, and ventral to the site. In SM's right hemisphere, voxels immediately surrounding the lesion lacked object-selectivity. Object-selective voxels were exclusively found approximately 5 mm posterior to the lesion. In SM's left hemisphere, no object-selective responses were found in mirror-symmetric locations. Our data suggest that the right medial fusiform gyrus is critically involved in causing object agnosia and, furthermore, in adversely affecting object processing in structurally intact areas of the ventral pathway in the non-lesioned hemisphere. Future studies will show the impact of this isolated lesion on object processing in the dorsal pathway.